Hydraulic tensioners incorporating check valves have been widely used to maintain proper tension, and to suppress vibration, in a timing belts or timing chain used to transmit rotation between a crankshaft and one or more camshafts in a vehicle engine.
As shown in FIG. 12, a conventional hydraulic tensioner 500 is typically mounted on an engine adjacent the slack side of a timing chain C, which is driven by a crankshaft sprocket S1 and in driving relationship with camshaft sprockets S2. A hollow plunger 520 protrudes slidably from a housing 510 and applies tension to the slack side of the timing chain C by pressing against the back of a pivoted lever L1 on which the chain slides. A fixed guide L2 is provided on the tension side of the timing chain C. The sprockets and chain move in the directions indicated by arrows in FIG. 12.
As shown in FIG. 13, in the hydraulic tensioner 500, the cylindrical plunger 520 fits slidably in a plunger-accommodating hole 511 formed in the housing 510. A high pressure oil chamber R is formed by the plunger 520 and a plunger-accommodating hole 511. The plunger is urged in the protruding direction by a plunger-biasing coil spring 530.
A check valve unit 540 is press-fit into the bottom portion of the plunger-accommodating hole 511. The check valve unit allows oil to flow from a source (not shown) of oil under pressure into the high pressure oil chamber R, but blocks reverse flow of oil.
The check valve unit 540 comprises a ball 541, a ball guide 542, which envelops the ball 541, a retainer 543, fixed to one end of the guide 542, and a ball seat 544, fixed to the opposite end of the guide 542. The ball can move toward and away from the seat through a distance limited by the retainer. When the ball guide is moved away from the seat, oil can flow through the check valve unit 540 into the high pressure oil chamber R. When the ball is in engagement with the seat, it blocks reverse flow of oil.
In operation of the tensioner, oil in the high pressure oil chamber R leaks through a slight clearance between the outer circumferential surface of the plunger 520 and the inner circumferential surface of the plunger-accommodating hole 511, and is discharged to the outside of the housing 510. Because of the viscosity of the oil, there is a resistance to flow through the clearance between the plunger and the plunger-accommodating hole. The resistance to flow enables the tensioner to exert a damping action, absorbing impact forces exerted on the plunger 520 and reducing vibration of the plunger 520. An example of a hydraulic tensioner having the above-described features is found in United States Patent Application Publication US2005/0227799.
In a conventional hydraulic tensioner, oil is supplied to the high pressure oil chamber by a pump driven by an engine. When the engine is stopped, the supply of oil to the high pressure oil chamber is also stopped. Some of the oil left in the chamber leaks through the clearance between the plunger and the inner circumferential surface of the plunger-accommodating hole and is discharged and replaced by air. When the engine is re-started after having been stopped for a long time, a considerable amount of time is required for replenishment of the oil in the high pressure oil chamber of the tensioner, and the damping action of the tensioner is therefore delayed.
The delay between the time the engine is started and the time at which the damping action of the tensioner commences can be reduced by shortening the high pressure oil chamber or reducing the thickness of the plunger-biasing spring. However, these measures reduced the load capacity and spring constant of the spring, and impair the performance of the tensioner.
The invention solves the above-described problems by providing a hydraulic tensioner in which the time interval required to replenish oil in the high pressure oil chamber following engine start-up can be significantly reduced without the need to change the properties of the plunger-biasing spring.